Evolution-Data Optimized (EVDO) is a wireless radio broadband data standard evolved directly from the cdma2000 single-carrier Radio Transmission Technology (1xRTT or simply 1x) standard that operates over the Code Division Multiple Access (CDMA) network infrastructure. EVDO comprises an Internet Protocol (IP)-based network and Quality of Service (QoS) mechanisms to support applications that rely on quick set-up and low-latency transmissions over packet-based networks. A recent revision of the EVDO standard, EVDO Revision A, offers higher data rates and higher system capacity, and provides additional QoS support that significantly reduces latency and improves performance of delay-sensitive applications. EVDO RevA may be particularly useful to support minimum QoS communication sessions, such as voice over Internet Protocol (VoIP) calls, and data uploads and downloads with QoS specifications.
A VoIP call over EVDO RevA requires certain minimum QoS guarantees at a traffic channel established between an access terminal (i.e., a wireless communication device) and an access network for the call to sound at least as good as a circuit switched voice call over the 1x standard. QoS guarantees may include limits on the amount of packet loss, delay, and jitter that each data flow within the VoIP call may experience. For example, a VoIP call may include a signaling data flow and a media data flow associated with respective Radio Link Protocol (RLP) flows.
At a particular time or place, if the access network does not have enough resources to guarantee the minimum QoS for a VoIP call with an access terminal, a communication application on the access terminal may get a notification from the protocol stack software that QoS is suspended. The communication application may then drop the VoIP call and may instead try a circuit switched voice call over the 1x standard. In addition to resources on the access network, the QoS for a VoIP call is also dependent on the state of the connection between the access terminal and the access network. If the connection at the radio layer is down, the communication application gets the same notification that QoS is suspended and drops the VoIP call. The reasoning for this is that an access terminal that is not even connected at the radio layer should not unnecessarily take up radio resources of the access network.
A hard handoff of the access terminal between a first access network and a second access network during a VoIP call may cause the connection at the radio layer to go down momentarily while the communication session is being transferred from the first access network to the second access network. Therefore the communication application receives notification that QoS for the VoIP call is suspended, and drops the VoIP call. However, the connection at the radio layer is typically down only temporarily and, if the hard handoff is successful, the connection may be up again with the second access network in a short time.